A structure to reduce a rattling noise caused by a gear rattle has been proposed in this type of electric power steering device. In detail, an eccentric cam mechanism is disposed at an end of a worm shaft of a decelerator. The rattling noise caused by the gear rattle is reduced by adjusting a backlash between the worm shaft and a worm wheel while increasing or decreasing the center-to-center distance therebetween.
However, the worm shaft is displaced in a direction parallel to the axis of the worm wheel, and therefore, the center position of engagement therebetween deviates. Therefore, variations in tooth contact occur, and therefore, disadvantageously, an operating noise becomes louder, or durability is lowered.
Hence, an electric power steering device of Patent Document 1 has been proposed. The electric power steering device of Patent Document 1 includes, at a lower position in an axial direction of the worm wheel, a pair of bearings that support a lower end of an output shaft and an eccentric bearing holder that supports the pair of bearings. The center-to-center distance therebetween is increased or decreased by moving the lower end of the output shaft and the worm wheel in a radial direction while rotating the eccentric bearing holder.
Additionally, an electric power steering device of Patent Document 2 has been proposed. The electric power steering device of Patent Document 2 includes a worm-shaft-side housing that supports a worm shaft and a motor and includes first and second housings that hold the worm-shaft-side housing between upper and lower sides in an axial direction of a worm wheel. At least one of the first and second housings supports the worm wheel. The electric power steering device of Patent Document 2 additionally includes a backlash adjustment mechanism that adjusts the center-to-center distance between the worm shaft and the worm wheel while relatively moving the worm-shaft-side housing with respect to the first and second housings. In detail, the backlash adjustment mechanism rotates a camshaft having an eccentric cam in a through-hole through which the worm-shaft-side housing and the first and second housings are inserted in parallel with the axial direction of the worm wheel.